Forum Icon Colors

Every time I click on the Forums Tab the little picture icon next to each board is black and white. The next day I go on they have color on them. Or sometimes half of them are gray and the rest are colored. Why is this?

"As you probably have also noticed a black & white icon means no new posts have been made, while a colored icon appears when there are new posts since you last visit on that specific board." -- http://www.techspot.com/vb/topic127953.html